Add health check to checkpointing

Take action if we are running out of checkpoint space.
Configurable via ro.sys properties.
ro.sys.cp_usleeptime = Time to sleep between checks
ro.sys.cp_min_free_bytes = Min free space to act on
ro.sys.cp_commit_on_full = action to take. Either commits or reboots to
                           continue attempt without checkpoint, or retry
			   and eventually abort OTA

Test: Trigger a checkpoint and fill the disk.
Bug: 119769392
Change-Id: I977cc03b7aef9320d661c8a0d716f8a1ef0be347